1. Field of the Invention
The present invention relates to matrix substrates used in liquid crystal devices and to liquid crystal display devices using the matrix substrates.
2. Description of the Related Art
Recent progress in information networks increasingly requires display devices for communication of information, particularly image information. Liquid crystal display devices, which are thin and have an advantage in low electrical power consumption, have attracted considerable attention and are growing as one of the basic industries, similarly to the semiconductor industries. Recently, liquid crystal display devices are mainly used in 12xe2x80x3 notebook personal computers. In the future, liquid crystal display devices having larger screen sizes will be used in workstations and home televisions, as well as in personal computers. Trends of increasing scale in liquid crystal display devices, however, demands the introduction of expensive apparatuses for producing such devices. Further, large scale liquid crystal display devices must have extreme electrical characteristics for driving large screens. Thus, production costs increase significantly, that is, are in proportion to from the square to the cube of the screen size.
A front- or rear-projection system using a small liquid crystal display panel has recently attracted attention in which a liquid crystal image is optically enlarged and displayed. Performance and production costs of liquid crystal display devices are improved with size reduction in the devices by the scaling rule as in semiconductors. In TFT liquid crystal display panels, TFTs using polycrystalline Si are being substituted for those using amorphous Si to meet the requirement of small TFTs having high driving force. Image signals having a resolution level in the NTSC standard do not require high-speed processing.
A possible TFT liquid crystal display device which can be used to meet such requirements has an integrated structure including a display region and peripheral driving circuits, such as a shift register and a decoder, which are also formed of polycrystalline Si. Polycrystalline Si, however, is not comparable to single crystal Si. When a display of an extended graphics array (XGA) or super extended graphics array (SXGA) class in the resolution standard of computers is designed, for example, the shift register must inevitably be divided into a plurality of segments. Signal noise (ghosting) will occur in the display region corresponding to the boundary between the segments. Countermeasures are required for solving such problems.
Display devices using single-crystal Si substrates have attracted attention in place of integrated polysilicon display devices, since transistors in their peripheral driving circuits have significantly high driving characteristics, and thus, the single-crystal devices do not require divisional arrangements which are essential for polysilicon display devices. Signal noise due to the divisional arrangements does not occur in single-crystal devices.
The present inventors have disclosed reflection-type liquid crystal display devices using a poly-crystalline substrate and a single crystal Si substrate in Japanese Patent Laid-Open No. 9-73103. The technology solves a problem of reduction in light reflectance by random scattering at pixel electrodes having uneven surfaces and a reduction in contrast by unsatisfactory alignment of the orientation film in the rubbing step and thus by insufficient alignment of the liquid crystal which is caused by such uneven surfaces. Chemical mechanical polishing (hereinafter referred to as CMP) is employed to form all pixel electrodes each having a mirror surface in the same plane. Thus, this reflection-type liquid crystal display device, free of random light scattering and insufficient alignment, can display high-quality images.
The method for making an active matrix substrate for the reflection-type liquid crystal display device disclosed in Japanese Patent Laid-Open No. 9-73103 will now be described with reference to FIG. 32. Although FIG. 32 shows a pixel region, peripheral driving circuits such as shift registers for driving switching transistors in the pixel region can also be formed on the same substrate.
An n-type silicon semiconductor substrate 201 having an impurity concentration of 1015 cmxe2x88x923 or less is subjected to local thermal oxidation to form a LOCOS (local oxidation of silicon) layer 202, and boron ions are implanted in a dose of approximately 1012 cmxe2x88x922 through the LOCOS layer 202 as a mask to form a PWL 203 being a p-type impurity region having an impurity concentration of 1016 cmxe2x88x923. The substrate 201 is thermally oxidized to form a gate oxide film 204 having a thickness of 1,000 angstroms or less (FIG. 32A).
An n-type polysilicon gate electrode 205 is formed by doping phosphorus in an amount of approximately 1020 cmxe2x88x923; phosphorus ions are implanted onto the entire surface of the substrate 201 in a dose of approximately 1012 cmxe2x88x922 to form an NLD 206 being an n-type impurity region having an impurity concentration of 1016 cmxe2x88x923. Phosphorus ions are implanted through a patterned photoresist mask at a dose of approximately 1015 cmxe2x88x922 to form source and drain regions 207 and 207xe2x80x2 having an impurity concentration of approximately 1019 cmxe2x88x923 (FIG. 32B).
A phospho-silicate glass (PSG) film 208, which is a phosphorus-doped oxide film, is formed as an interlayer on the entire substrate 201. The PSG film 208 can be replaced with a nondoped silicate glass (NSG)/boro-phospho-silicate glass (BPSG) film or a tetraethoxysilane (TEOS) film. Contact holes are patterned into the PSG film 208 just above the source and drain regions 207 and 207xe2x80x2. Aluminum is deposited by a sputtering process and then patterned to form an aluminum electrode 209 (FIG. 32C). It is preferred that a barrier metal composed of Ti or TiN be formed between the aluminum electrode 209 and the source and drain regions 207 and 207xe2x80x2 so as to improve the ohmic contact characteristics between the aluminum electrode 209 and the source and drain regions 207 and 207xe2x80x2.
A plasma SiN film 210 with a thickness of approximately 3,000 angstroms, and then a PSG film 211 with a thickness of approximately 10,000 angstroms, are formed on the entire substrate 201 (FIG. 32D). The PSG film 211 is patterned using the plasma SiN film 210 as a dry etching stopper layer so as to leave the separation region between pixels, and then a thorough hole 212 is patterned just above the aluminum electrode 208 which is in contact with the drain region 207xe2x80x2 by dry etching (FIG. 32E).
A pixel electrode 213 with a thickness of approximately 10,000 angstroms or more is formed on the substrate 201 by sputtering or electron beam (EB) deposition (FIG. 32F). The pixel electrode 213 is composed of a metal film of aluminum, titanium, tantalum or tungsten, or a metal compound film of such a metal. The surface of the pixel electrode 213 is polished by CMP (FIG. 32G).
An alignment film 215 is formed on the resulting active matrix substrate, and its surface is subjected to alignment treatment such as rubbing. The substrate is bonded with a counter substrate with a spacer (not shown in the drawing) therebetween, and a liquid crystal 214 is injected into the gap to form a liquid crystal device (FIG. 32H). The counter electrode includes a transparent substrate 220, a color filter 221, a black matrix 222, a common electrode composed of ITO, and an alignment film 215xe2x80x2, in that order.
The reflection-type liquid crystal device is driven as follows. Peripheral circuits including a shift register which is formed on the substrate 201 by an on-chip process applies a signal potential to the source region 207 and a gate potential to the gate electrode 205 such that the switching transistor in the pixel in an ON state supplies signal charge to the drain region 207xe2x80x2. The signal charge is accumulated in a pn-junction cavity capacitor formed between the drain region 207xe2x80x2 and the PWL 203 to impart a potential to the pixel electrode 213 through the aluminum electrode 209. The potential application to the gate electrode 205 is suspended when the potential of the pixel electrode 213 reaches a given value so that the pixel switching transistor is in an OFF state. The signal charge accumulated in the pn-junction capacitor fixes the potential of the pixel electrode 213 before the pixel switching transistor is redriven. The fixed potential of the pixel electrode 213 drives the liquid crystal 214 encapsulated between the substrate 201 and the counter substrate 220 shown in FIG. 32H.
The pixel electrode 213 of the active matrix substrate has a smooth surface as shown in FIG. 32H, and an insulating layer is embedded into the gap between two adjacent pixel electrodes. Thus, the alignment film 215 formed thereon has a smooth surface, which prevents a decrease in light efficiency due to light scattering, a decrease in contrast due to insufficient rubbing, and the formation of an emission line due to a horizontal electric field formed by a step between two pixel electrodes. As a result, the quality of the displayed image is improved.
This reflection-type liquid crystal display device, however, requires further improvements, according to the research by the present inventors. In the reflection-type liquid crystal display device, two adjacent reflecting electrodes are connected to each other by capacitive coupling. The capacitance increases in some cases when the distance between the two adjacent electrodes is increased to increase the aperture ratio. Thus, the potential of the reflecting electrode significantly changes with a change in the potential of the adjacent reflecting electrodes by capacitive coupling and the capacitance between the reflecting electrodes with the capacitance of the pixel itself.
Accordingly, it is an object of the present invention to provide a reflection-type liquid crystal display device having a high aperture ratio which stabilizes the pixel potential, precisely controls the voltage applied to the liquid crystal, and displays a high-quality image.
A first aspect of the present invention is a matrix substrate comprising a plurality of conductive members constituting pixels provided on a substrate, the plurality of conductive members forming substantially a smooth plane, wherein the matrix substrate further comprises a nonconductive section for insulating the plurality of conductive members from each other, the distance between two adjacent conductive members being lower at the surfaces of the conductive members than at the side of the substrate.
A second aspect of the present invention is a liquid crystal display device comprising a matrix substrate having conductive members constituting pixels provided on a substrate, the plurality of conductive members forming substantially a smooth plane, a counter substrate, and a liquid crystal material placed between the plurality of conductive members and the counter substrate, wherein the matrix substrate further comprises a nonconductive section for insulating the plurality of conductive members from each other, and the distance between two adjacent conductive members is lower at the surfaces of the conductive members than at the side of the substrate.
In the present invention, the distance between two adjacent conductive members is lower at the surfaces of the conductive members than at the side of the substrate. Thus, the liquid crystal display device has a large aperture ratio, and fluctuation of the pixel voltage by capacitive coupling between two adjacent pixels is reduced. At a result, the voltage applied to the liquid crystal is precisely controlled and high-quality images are displayed.
Preferably, the ratio H/L of the height of the nonconductive members H to the distance between the two adjacent conductive members H is in a range of 0.2 to 1.0. The yield of the device is thereby improved, and the advantages of the present invention are further improved.
Preferably, in the projection-type liquid crystal display device, one of the plurality of microlenses is provided for each three pixels. High-quality images free of mosaic patterns are thereby displayed.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.